How can I build a database-based Spring Boot environment / property source? - spring

Question

The goal is running Spring Boot application with an Environment containing keys & values loaded and generated by a database connection (DataSource).
Or, more abstract defined: While a configuration by files only should be preferred (faster, easier, more tolerant, ...), sometimes you will find use cases where a non-static files based configuration is required.
Spring 3.1 introduces Environment which is actually a property resolver (extends PropertyResolver) and is based on a list of objects PropertySource. Such a source is a wrapper/adapter for a properties (file or object), a map or something else. It really looks like this is the way how to get.
Properties properties = new Properties();
properties.put("mykey", "in-config");
PropertiesPropertySource propertySource = new PropertiesPropertySource("myProperties", properties);
However, this cannot be done in #Configuration classes since it must be available for the configuration phase. Think about something like
#Bean public MyService myService() {
if ("one".equals(env.getProperty("key")) {
return new OneService();
} else {
return new AnotherService();
}
}
// alternatively via
#Value("${key}")
private String serviceKey;
Additionally, the more recent Spring releases support Condition as well.
With a OneCondition like
public class OneCondition implements Condition {
#Override
public boolean matches(final ConditionContext context, final AnnotatedTypeMetadata metadata) {
return "one".equals(context.getEnvironment().getProperty("key"));
}
}
This can be used like
#Bean
#Conditional(OneCondition.class)
public MyService myService() {
return new OneService();
}
My non working ideas:
Option 1: #PropertySource
The corresponding annotation processor handles files only. This is fine, but not for this use case.
Option 2: PropertySourcesPlaceholderConfigurer
An example with a custom property source would be
#Bean
public static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() throws IOException {
PropertySourcesPlaceholderConfigurer pspc = new PropertySourcesPlaceholderConfigurer();
pspc.setIgnoreUnresolvablePlaceholders(Boolean.TRUE);
// create a custom property source and apply into pspc
MutablePropertySources propertySources = new MutablePropertySources();
Properties properties = new Properties();
properties.put("key", "myvalue");
final PropertiesPropertySource propertySource = new PropertiesPropertySource("pspc", properties);
propertySources.addFirst(propertySource);
pspc.setPropertySources(propertySources);
pspc.setLocations(new PathMatchingResourcePatternResolver().getResources("classpath*:application.properties"));
return pspc;
}
However, this only configures the placeholders (i.e. #Value. Any environment.getProperty() will not profit.
This is more or less the same as Option 1 (less magic, more options).
Do you know a better option? Ideally, the solution would use the context datasource. However, this is conceptually an issue since the datasource bean creation relies on properties itself...

Answer 1

Spring Boot provides some different extensions point for this early processing step: http://docs.spring.io/spring-boot/docs/current/reference/html/howto-spring-boot-application.html#howto-customize-the-environment-or-application-context
Internally, these options are realised with implementations of standard Spring ApplicationContextInitializer.
Depending on the priority of the source, the key/value will be available both in environment.getProperty() as well in property placeholders.
Because these is a pre-config-context listeners, no other beans are available, like a DataSource. So if the properties should be read from a database, the datasource and connection have to be build manually (eventually a separated datasource connection lookup).
Option: ApplicationListener for ApplicationEnvironmentPreparedEvent
Build an implementation of an application listener consuming ApplicationEnvironmentPreparedEvents and
register it in META-INF/spring.factories and the key org.springframework.context.ApplicationListener
- or -
use the SpringApplicationBuilder:
new SpringApplicationBuilder(App.class)
.listeners(new MyListener())
.run(args);
Example
public class MyListener implements ApplicationListener<ApplicationEnvironmentPreparedEvent> {
#Override
public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
final ConfigurableEnvironment env = event.getEnvironment();
final Properties props = loadPropertiesFromDatabaseOrSo();
final PropertiesPropertySource source = new PropertiesPropertySource("myProps", props);
environment.getPropertySources().addFirst(source);
}
}
Reference: http://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-spring-application.html#boot-features-application-events-and-listeners
Option: SpringApplicationRunListener
Besides the special event, there is also a more general event listener containing hooks for several types of events.
Build an implementation of SpringApplicationRunListener and register it in META-INF/spring.factories and the key org.springframework.boot.SpringApplicationRunListener.
Example
public class MyAppRunListener implements SpringApplicationRunListener {
// this constructor is required!
public MyAppRunListener(SpringApplication application, String... args) {}
#Override
public void environmentPrepared(final ConfigurableEnvironment environment) {
MutablePropertySources propertySources = environment.getPropertySources();
Properties props = loadPropertiesFromDatabaseOrSo();
PropertiesPropertySource propertySource = new PropertiesPropertySource("myProps", props);
propertySources.addFirst(propertySource);
}
// and some empty method stubs of the interface…
}
Reference: http://docs.spring.io/spring-boot/docs/current/reference/html/howto-spring-boot-application.html#howto-customize-the-environment-or-application-context
Option: ApplicationContextInitializer
This is an old friend for all "non Boot" Spring developers. However, SpringApplication mocks a configuration away -- at first.
Build an implementation of ApplicationContextInitializer and
register it in META-INF/spring.factories and the key org.springframework.context.ApplicationContextInitializer.
- or -
use the SpringApplicationBuilder:
new SpringApplicationBuilder(App.class)
.initializers(new MyContextInitializer())
.run(args);
Example
public class MyContextInitializer implements ApplicationContextInitializer {
#Override
public void initialize(final ConfigurableApplicationContext context) {
ConfigurableEnvironment environment = context.getEnvironment();
MutablePropertySources propertySources = environment.getPropertySources();
Properties props = loadPropertiesFromDatabaseOrSo();
PropertiesPropertySource propertySource = new PropertiesPropertySource("myProps", props);
propertySources.addFirst(propertySource);
}
}